The instant invention is directed to an instrument and method of measuring the concentration of a target element in a multi-layer, thin coating and, more particularly, to such an instrument and method that is capable of measuring non-destructively the lead concentration on a surface coating.
Lead paint was used widely around 1920 due to its durability. Later, lead pigments were found to be a health hazard. It was banned from use since 1978. Old buildings need to be checked for lead paint to make sure it is safe for kids, thus it is necessary to have a non-destructive method for detecting the lead concentration from a surface.
A lead K line based instrument can be used to measure the lead concentration. K lines have very high energy (75 keV and 85 keV). They can travel through many layers of covering material with very little loss. Thus, by simply measuring the K line x-ray intensity, lead concentration can be determined. However, many drawbacks are associated with K line based instruments. Thus:                (a) First, K line x-ray intensity is very weak and the measurement error can be large (when the test time is fixed). When the lead concentration is low (around 1 mg/cm2), the large statistical error makes the reading unreliable.        (b) Even if the lead paint is on the opposite side of the wall or is covered by another layer of dry wall (after deleading), lead can still be detected, erroneously reporting a lead hazard when it is totally safe.        (c) A radioactive isotope source is present (needed to generate high energy x-rays to excit lead K shell electrons).        (d) L line x-rays have much less energy. Its escape depth is much shorter. Ten layers of regular paints on top of lead paint will reduce the lead L line intensities by more than 90%. Thus an L line based instrument is a surface lead detector. For single-layered lead paint, there is a well established method for measuring the surface lead concentration from lead Lα and Lβ lines (as described in U.S. Pat. No. 5,274,688 and U.S. Pat. No. 5,396,529). The lead paint can be on top of the surface or buried by non-lead containing material, and this method works accurately. However, when the paint contains multiple layers of lead paint, the method fails badly.        (e) The current action level for deleading is 1.0 mg/cm2 and the trend is moving lower. Thus the most critical lead range is from 0 to 1.5 mg/cm2. In this range, the large statistic error of K line based reading is a real problem. On the other hand, buildings that are 100 years old are likely to have multiple layers of lead paint, the limitation of the lead L line based prior art makes the instrument based on the technology useless for those buildings.        
The present disclosure provides a solution that targets the needs of and meets the following primary objectives:                (a) Reliably and non-destructively determining whether or not the amount of lead is less than the action level (current action level is 1.0 mg/cm2);        (b) Handling both single layer and multi-layer lead paint coatings; and        (c) Operate without using a radioactive source.        